For recording media represented by CD (Compact Disc), Digital Versatile Disc (DVD), BD (Blu-ray Disc), and the like, increased recording density has been mostly dealt with to short the wavelength of laser light and increasing the NA (numerical aperture) of an objective lens. However, both of these measures are said to approach the limits for technical reasons or the like.
Various proposals have been made to break down this situation; every effort has been made to develop a method for forming a recording layer of multiple layers in order to increase the recording density. This method is an extension of the current optical disc technique and is considered to be very practical. However, there are growing concerns about a decrease in productivity associated with multilayered optical discs. A reason for the concerns is that tracking grooves containing address information need to be formed for each layer. The need for a molding operation for each layer increases processing time or reduces manufacturing yield.
In connection with this, a multilayered optical disc based on what is called a guide layer (GL system) is known which includes one guide layer and a plurality of information recording layers so that grooves for tracking are provided only in the guide layer. The optical disc with the guide layer different from the information recording layers is advantageous in that molding processing needs to be carried out only on the guide layer, preventing an increase in the number of molding processing operations in spite of an increased number of information recording layers resulting from a higher capacity.
For optical discs based on the GL system, recording is carried out as follows. First, light with a DVD wavelength is concentrated on the guide layer using an objective lens in order to perform tracking based on guide grooves provided in the guide layer. Then, recording is carried out by using the same objective lens to concentrate light with a BD wavelength for recording on the recording layer. The GL system is characterized in that the guide layer is responsible for the tracking operation for recording.